1. Field of the Invention
The present invention relates to a method for heat-treating a hollow cylindrical workpiece. The hollow cylindrical workpiece includes, for example, a bushing which is one of members used for an endless track mounted to construction vehicles, but the hollow cylindrical workpiece is not limited to the bushing only.
2. Description of Related Art
For a hollow cylindrical work such as a bushing used for an endless track, hardness is required at an inside surface, an outside surface, and portions adjacent thereto so as to ensure a high wear resistance, and toughness is required at a core portion of a wall so as to prevent cracks from propagating from the surfaces.
In order to satisfy those two quality requirements at the same time, the following heat-treatment methods have been proposed:
(1) A method as proposed in Japanese Patent Publication No. SHO 59 - 77979, as illustrated in FIG. 6, includes a first step in which a workpiece 111 is subjected to high-frequency quench-hardening from an outside surface of the workpiece 111 to a part of an inside effective quench-hardened layer, and a second step in which, after the first step, an inside portion of the workpiece 111 is subjected to high-frequency quench-hardening, while a core portion of a wall of the workpiece 111 is tempered.
In FIG. 6, a reference numeral 112 denotes a tool for setting a workpiece, reference numeral 113 denotes a heating coil, reference numeral 114 denotes a cooling jacket, reference numeral 115 denotes a heating coil, and reference numerals 116 and 117 denote cooling jackets. In the first step, the workpiece 111 is conveyed in a vertical direction, and quench-hardening is conducted by heating the workpiece 111 from the outside surface, followed by cooling the workpiece 111 from the outside surface. In the second step, the workpiece 111 is conveyed in a vertical direction, and quench-hardening is conducted by heating the workpiece 111 from the inside surface, followed by cooling the workpiece 111 from the outside surface and from the inside surface at the same time.
FIG. 7 illustrates a hardness distribution in the workpiece 111 after each step has been conducted.
(2) A method as proposed in Japanese Patent Publication No. HEI 9-143564 (Japanese Patent Application No. HEI 7-299997), as illustrated in FIG. 8, includes a first step in which high-frequency quench-hardening is conducted to an entire cross-section of a wall from an outside surface to an inside surface of a workpiece 120, and a second step in which, after the first step, an inside portion of the workpiece 120 is subjected to high-frequency quench-hardening, while a core portion of the wall is tempered.
In FIG. 8, reference numeral 121 denotes a heating coil, reference numeral 122 denotes a cooling jacket, reference numerals 123 and 124 denote rotatory rollers, reference numeral 125 denotes a heating coil, and reference numeral 126 denotes a cooling jacket. In the first step, the workpiece 120 is conveyed in a horizontal direction, and quench-hardening is conducted by heating the workpiece 120 from the outside surface, followed by cooling the workpiece 120 from the outside surface after a temperature of the workpiece 120 is made uniform, while the workpiece 120 is conveyed from the heating coil 121 to the cooling jacket 122 which are spaced from each other. In the second step, the workpiece 120 is conveyed in a vertical direction including an oblique direction inclined with respect to the vertical direction, and quench-hardening is conducted by heating the workpiece 120 from the inside surface, followed by cooling the workpiece 120 from the outside surface.
FIG. 9 illustrates a hardness distribution in the workpiece 120 after each step has been conducted.
(3) A method as proposed in Japanese Patent Publication No. HEI 1-75629 includes a first step in which high-frequency quench-hardening is conducted to an entire cross-section of a wall from an outside surface to an inside surface of a workpiece, and a second step in which after the first step, while an inside portion of the workpiece is cooled, the workpiece is high-frequency quench-hardened from the outside portion, and a core portion of the wall is tempered.
In the second step, a cooling jacket is inserted within an inside surface of the workpiece so as to cool the inside portion of the workpiece.
However, in the above conventional methods, it is not possible to conduct a continuous heat-treatment in an integrated line including the first step and the second step.
More particularly, with the first conventional method (1), since the workpiece is conveyed in a vertical direction in both the first step and the second step, heat-treating the workpieces is not conducted continuously, but conducted intermittently one by one.
With the second conventional method (2), in the first step, the workpiece is conveyed in a horizontal direction, which enables a continuous heat-treatment in the first step. However, since the workpiece is conveyed in a vertical direction in the second step, heat-treating the workpieces is conducted intermittently one by one in the second step.
With the third conventional method (3), since the cooling jacket is inserted within the inside surface of the workpiece in the second step, heat-treating the workpieces is conducted intermittently one by one.
Accordingly, the above conventional methods have the following problems:
(a) A total cycle time is determined by a cycle time of the intermittent heat-treatment step, so that productivity is not further improved beyond that. PA1 (b) A handling apparatus for inserting the heating coil and the cooling jacket within the inside surface of the workpiece needs to be provided, resulting in an increase in an initial cost. PA1 (c) Since a temperature difference is caused between opposite end portions and a central portion of an entire length of the workpiece in the intermittent heat-treatment, the magnitude of deviation in a dimension of the workpiece after heat-treatment is increased.